Contact ionization ion source

ABSTRACT

An ion source in which an apertured or foraminous electrode having a multiplicity of openings is spaced from one or more active surfaces of an ionisation electrode, the active surfaces comprising a material capable of ionising by contact ionization a substance to be ionized supplied during operation to the active surface or surfaces comprises means for producing during operation a magnetic field which enables a stable plasma to be formed in the space between the active surface or surfaces and the apertured electrode, the field strength of the magnetic field being preferably in the range between 2 and 8 kilogauss.

United States Patent Hashmi et al. Feb. 4, 1975 [54] CONTACT IONIZATION ION SOURCE 2,831,996 4/1958 Martina 313/63 1 Inventors: Nujeeb Heshmi, Kefigsbeeerstrasse $15235 $51329 52353511211167":1133;133:1332: 33122 g gz=;g Van 3,479,545 11/1969 Wilson et al 313/63 Oneginstrasse 8, Munich, both of I e y Primary ExammerW1ll1am F. L1ndqu1st Attorney, Agent, or Firm-Flynn & Frishauf [22] Filed: May 31, 1973 [21] Appl. No.. 365,621 ABSTRACT Related US. Application Data [63] Continuation of Ser- 162,489 My 14 1971 An 1on source 1n which an apertured or forammous abandoned. electrode having a multiplicity of openings is spaced from one or more active surfaces of an ionisation elec- 30 Foreign A li ti priority Data trode, the active surfaces comprising a material capa- Juy 25 1970 Germany 2037029 ble of 1on1smg by contact ionizatlon a substance to be ionized supplied during operation to the active surface [52 us. 01 250/423 250/425 313/63 P i Pmd"cing during 151 1111. C1. noij 39/34 a magnet'c wh'ch enables a Stable Plasma [58] Field of Search 250/423 424 425 427' be fmmed the Space the active 1 or surfaces and the apertured electrode, the field strength of the magnetic field being preferably in the [56] Reerences Cited range between 2 and 8 kilogauss.

UMTED STATES PATENTS 8 Claims, 2 Drawing Figures 2,806,161 9/1957 Foster, Jr 313/63 V PAIENIEBFEB 41975 INVENTORI BYI CONTACT IONIZATION ION SOURCE This is a continuation, of application Ser. No. 162,489, filed July 14, 1971, now abandoned.

The present invention concerns ion sources.

A known form of ion source has a heatable ionisation electrode which has an active surface of a material capable of ionising, by contact ionization, a substance which is to be ionized, a device for supplying atoms of the substance to be ionized to the active surface of the ionization electrode, and an open-structure electrode which can be biased negatively relative to the ionizing electrode and through which the ions produced pass to a consumer (see, for example, The Review of Scientific Instruments Vol. 40, No. 8, August 1969, pages 1072 to 1074.

In this known form of ion source, however, the attainable specific ion current strength, which is defined as the ion current strength per unit of active surface of ionization electrode, and the efficiency leave much to be desired, since in operation, a space charge is formed in front of the ionization electrode and impedes the supply of ions, and consequently the ions formed have to be extracted from the source by comparatively high electric fields.

According to one aspect of the present invention there is provided an ion source in which an openstructured electrode having a multiplicity of openings is spaced from one or more active surfaces of an ionization electrode, the active surfaces being formed of a material capable of ionizing by contact ionization a substance to be ionized supplied during operation to the active surface or surfaces, and means are provided for producing during operation a magnetic field which enables a stable plasma to be formed in the space between the active surface or surfaces and the openstructured electrode.

According to another aspect of the present invention there is provided an ion source having an ionization electrode with one or more active surfaces formed of a material capable of ionizing by contact ionization a substance to be ionized supplied thereto during operation, a grid electrode spaced from-the said active surface or surfaces, and means for producing during operation a magnetic field which enables a stable plasma to be formed in the space between the active surface or surfaces and the grid electrode.

The strength of the magnetic field is preferably greater than 1.5 or 2 kilogauss and less than 5 or 8 kilogauss. For reasons of economy, efforts should be made to manage with the lowest possible magnetic field strengths.

The distance between the active surface of the ionization electrode and the open-structured electrode, which may, for example, be in the form of a grid grating, lattice, or some other open network, amounts to at least 20 times the Debye length (6.9 T/n, where T is the temperature in degrees Kelvin and n is the number of particles per cubic centimetre in the plasma). In practice, the distance selected will be at least 10 cen timetres, and preferably it lies in the range between about 20 and 30 centimetres. A larger distance is inexpedient on account of the magnetic field.

The voltage between the ionization electrode and the open-structure electrode need be only relatively low, for example 10 to 20 volts, since it merely serves to hold back the electrons of the plasma. The high energy required in the known ion sources for drawing off the ions from the space charge is not needed in the present ion source.

A preferred field of application of the ion source according to the invention is in devices for isotope separation operating on the principle of a mass spectrometer. Such isotope separating devices, compared with isotope separating devices operating on other principles, are characterised by a very high separation factor. Hitherto, it has not been possible to achieve satisfactory mass throughput with such devices since ion;

be described with reference to the accompanying drawings, in which:

FIG. 1 shows a greatly simplified axial section of an ion source according to a first embodiment of the invention;

FIG. 2 shows a greatly simplified axial section of an ion source according to a second embodiment of the invention.

An ion source is shown in FIG. 1 arranged in a vacuum vessel 10 having a connecting piece 12 leading to a vacuum pump system of conventional type. The vacuum vessel 10 also has a second connecting piece 14 through which the ions produced are passed to a consumer 16, which may be an isotope-separating device operating on the principle of a mass spectrometer. (Such isotope separating devices are known; see for example the publication by Smith, Parkins and Forrester in the periodical The Physical Review, Second Series, Vol.72, No. 11, Dec. 1, 1947, pages 989 to 1002.)

Mounted in the vacuum vessel 10 is an ionization electrode 18 in the form, for example, of a'circular plane plate which can be heated by electron bombardment. The electrons are produced by an incandescent cathode 20 connected to a heating voltage source 22,

and are shot at the back of the plate by a negative ac-' celerating voltage U, of, for example, several hundred volts relative to the ionization electrode'l8.

The vacuum vessel 10 also comprises an evaporator device 24, which contains a supply of a substance 26 which is to be ionized and supplies a vapour beam 28 of particles of the substance 26 to an active surface of the ionization electrode 18. The evaporator device may, as shown, comprise an evaporator boat 30 provided with electrical connections and heated by direct passage of current.

The vacuum vessel 10 is surrounded by a cylindrical magnet coil 34 which produces a constant magnetic field B having a field strength of preferably 2 and 8 kilogauss extending substantially at right-angles to the active surface 18a of the ionization electrode 18 and to an open-structure'electrode 36 arranged at a distance from and parallel to the ionization electrode.

The open-structure electrode consists, for example, of a tungsten wire net, and is biased in operation negative relative to the ionization electrode 18 by a bias voltage U,,.

The ionization electrode 18 consists, at least on its active surface 18a, of a material which is capable of ionizing the substance 26 by contact ionization. This means that the ionization voltage of the atoms to be ionized must be comparable with the exit voltage of the material on the active surface 18a. For ionizing alkali metals and alkaline-earth metals, and also uranium and the like, therefore, it is possible to use an electrode of tungsten, rhenium or tantalum, for example. If certain varieties of ceramics which are conductive in the hot state are used for the ionization electrode 18 or its active surface 18a, elements having higher ionization potentials may be ionized. Beryllium oxide, zirconium oxide and/or thorium oxide ceramics are examples of such ceramics.

In operation, the ionization electrode 18 is heated to the working temperature, for example 2,500K, by electron bombardment, and the evaporator device 24 is put into operation so that the beam 28 containing atoms of the substance to be ionized is directed on to the active front surface 18a of the electrode 18. The atoms impinging on the surface 18a are ionized by contact ionization upon evaporating again from the surface 18a, and, with the electrons thermally emitted by the electrode 18, form a stable plasma held together by the axial magnetic field B. The ions emerging from the plasma pass through the open-structure electrode 36, which is negatively biased relative to the ionization electrode 18, to the connection 14 and to the consumer 16. The negative voltage on the open-structure electrode 36 need be only comparatively low, for example to volts, since the only purpose of this voltage is to hold back the electrons of the plasma. The ions passing through the electrode 36 as indicated by the arrows 38 can then be accelerated and separated in a manner known in the art.

The embodiment shown in FIG. 2 operates on the same principle as the embodiment described with reference to FIG. 1. Corresponding parts have therefore been provided with the same reference numerals to which a prime is added. For the sake of simplicity, the vacuum vessel is not shown.

The device for supplying the atoms to be ionized again comprises an evaporator device 24 with an evaporator boat which, however, has the form of a ring or part of a ring and is positioned at the sideof the ionization electrode 18' facing away from the electrode 36'. The ionization electrode 18' has openings 40 through which the vapour 28' of the substance to be ionized passes, the vapour being ionized by contact with walls of the openings 40.

As in FIG. 1, the ionization electrode 18 may be heated by electron bombardment, in which case the boat 30' can act as cathode, or it may be heated by direct passage of current, a central electrode 42 and a peripheral electrode 44 being provided for this purpose.

The diameter of the ionization electrode 18 or 18 is preferably at least 50 millimetres. The distance between the electrodes 18 and 36 or 18' and 36' is preferably at least 100 millimetres, advantageously it lies between about 200 and 300 millimetres.

The ionization electrode and evaporator device may also consist of a heatable hollow body which contains a supply of the substance to be ionized and evaporated and has openings corresponding to the openings 40 from which the evaporated material escapes, the vapour being ionized on the walls.

The examples described herein may be modified in a large variety of ways, without going beyond the scope of the invention. It is, however, essential that there be in front of the active surface of the ionization electrode a magnetic field of a strength such that a stable plasma can be maintained which is composed of the ions produced and the thermally emitted electrons. If the ionizing substance is .already ingaseous or vapour form, an evaporator device is obviously not necessary.

In a further embodiment example, the open-structure electrode has the form of a perforated cylinder surrounding the plasma, the ions escaping in the radial direction through the cylindrical electrode to the consumer. In this case, a somewhat higher voltage, for example, about 200 to -300 volts, needs to be applied to the open-structure electrode than in the embodiments described hereinbefore.

What we claim is:

1. An ion source for use in an evacuated vessel, said source comprising:

means for supplying a gaseous or vaporized ionizable material in said evacuated vessel;

an ionization electrode having at least one active surface of substantial area formed of a material capable of ionizing said ionizable material by contact therewith;

means for heating said ionization electrode;

an open-structured electrode having a multiplicity of openings therein and spaced from said active surface or surfaces by space free of intervening fieldaffecting structures;

means for producing and maintaining, in the space between said active surface or surfaces and said open-structured electrode, a magnetic field of a character favoring the formation in said space of a stable plasma, and

means for applying to said open-structured electrode an electro-negative potential barely sufficient to repel electrons issuing from said plasma and to favor movement through said electrode of ions issuing from said plasma.

2. An ion source as defined in claim 1, in which said open-structured electrode is a grid electrode, in which, further, said electro-negative potential is such as to provide an average field strength between 0.3 and 2 volts per centimeter in the space between surface or surfaces and said open-structured electrode, and in which the axis of said magnetic field is essentially perpendicular to said active surface or surfaces.

3. Au ion source as defined in claim 1, in which said open-structured electrode is a grid electrode, and said surface or surfaces and said open-structured electrode are so disposed that the field in the space therebetween is essentially unidirectional and essentially aligned in the direction of said magnetic field.

4. An ion source as defined in claim 1, in which said ionizable material is a vaporized material and in which evaporator means are provided within said vessel for vaporizing said ionizable material.

5. An ion source as defined in claim 4, in which said evaporator means is disposed immediately adjacent said space between said active surface or surfaces and said open-structured electrode.

6. An ion source as defined in claim 4, in which said ionization electrode is traversed by openings whose wall surfaces consist of said material capable of ionizing said ionizable material by contact therewith and in which, further, said evaporator means is disposed on the opposite side of said ionization electrode from said open-structured electrode and adapted to produce vapor for ionization in said openings or at said active surface beyond said openings.

7. An ion source as defined in claim 1, in which said material capable of ionizing said ionizable material by contact therewith is a substance selected from the group consisting of tungsten, rhenium, tantalum and ceramic materials that are electrically conductive at the operating temperature of said ionization electrode.

8. Apparatus for separation of ionizable isotopes adapted for operation on the principle of a mass spectrometer in an evacuated vessel, which apparatus includes:

an ion source comprising an ionization electrode having at least one active surface formed of a material capable of ionizing said isotopes by contact therewith;

means for introducing said isotopes in gaseous or vapor form in the space adjacent to said ionization suing from said plasma.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3864575 Dated February 4, 1975 Inventor) Muj eeb HASHMI and Adriaan Der Houven It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the first page of the patent, the inventor's first name should be changed frcm "Nujeeb" to Mujeeb Jigrxed amf sealed this 15th ay of April 1375.

ask;

FORM PO-IOSO (10-69) USCOMM-DC 60376-P69 v.5. GOVIINIINT rnm'rmc ornc: nu o-au-su, 

1. An ion source for use in an evacuated vessel, said source comprising: means for supplying a gaseous or vaporized ionizable material in said evacuated vessel; an ionization electrode having at least one active surface of substantial area formed of a material capable of ionizing said ionizable material by contact therewith; means for heating said ionization electrode; an open-structured electrode having a multiplicity of openings therein and spaced from said active surface or surfaces by space free of intervening field-affecting structures; means for producing and maintaining, in the space between said active surface or surfaces and said open-structured electrode, a magnetic field of a character favoring the formation in said space of a stable plasma, and means for applying to said open-structured electrode an electronegative potential barely sufficient to repel electrons issuing from said plasma and to favor movement through said electrode of ions issuing from said plasma.
 2. An ion source as defined in claim 1, in which said open-structured electrode is a grid electrode, in which, further, said electro-negative potential is such as to provide an average field strength between 0.3 and 2 volts per centimeter in the space between surface or surfaces and said open-structured electrode, and in which the axis of said magnetic field is essentially perpendicular to said active surface or surfaces.
 3. An ion source as defined in claim 1, in which said open-structured electrode is a grid electrode, and said surface or surfaces and said open-structured electrode are so disposed that the field in the space therebetween is essentially unidirectional and essentially aligned in the direction of said magnetic field.
 4. An ion source as defined in claim 1, in which said ionizable material is a vaporized material and in which evaporator means are provided within said vessel for vaporizing said ionizable material.
 5. An ion source as defined in claim 4, in which said evaporator means is disposed immediately adjacent said space between said active surface or surfaces and said open-structured electrode.
 6. An ion source as defined in claim 4, in which said ionization electrode is traversed by openings whose wall surfaces consist of said material capable of ionizing said ionizable material by contact therewith and in which, further, said evaporator means is disposed on the opposite side of said ionization electrode from said open-structured electrode and adapted to produce vapor for ionization in said openings or at said active surface beyond said openings.
 7. An ion source as defined in claim 1, in which said material capable of ionizing said ionizable material by contact therewith is a substance selected from the group consisting of tungsten, rhenium, tantalum and ceramic materials that are electrically conductive at the operating temperature of said ionization electrode.
 8. Apparatus for separation of ionizable isotopes adapted for operation on the principle of a mass spectrometer in an evacuated vessel, which apparatus includes: an ion source comprising an ionization electrode having at least one active surface formed of a material capable of ionizing said isotopes by contact therewith; means for introducing said isotopes in gaseous or vapor form in the space adjacent to said ionization electrode; means for heating said ionization electrode; an open-structured electrode having a multiplicitY of openings therein and spaced from said active surface or surfaces by space free of intervening structures; means for producing and maintaining, in the space between said active surface or surfaces and said open-structured electrode, a magnetic field of a character favoring the formation in said space of a stable plasma, and means for applying to said open-structured electrode an electro-negative potential barely sufficient to repel electrons issuing from said plasma and to favor movement through said electrode of ions issuing from said plasma. 